The present invention relates to microelectrodes for placement within living beings to stimulate neurons electrically and to detect electrical neural activity, and relates particularly to the definition of electrode contact surfaces for such electrodes.
For several years research has been conducted in attempts to establish communication through living neurons, to communicate to the human brain information which can no longer be provided by a person's own eyes or ears, to stimulate paralyzed muscles, to stimulate autonomic nerves, as to control bladder function or pace the heart, or to control prosthetic limbs.
It is well known that electrical stimulation of certain nerves and certain regions of the brain can be perceived consciously, and research is being performed with the intention of eventually learning how to stimulate nerves in ways which can provide useful information to a person whose ability to hear or to see has been lost.
To utilize neural prostheses, electrical connections must be made to living neurons. Such connections must be made by extremely small electrodes, in order to isolate currents within small regions of living tissue. Active electrode sites can be placed very close to nerve cells, and electrical activity at the active electrode sites can be used to provide stimulation to the nerves. To limit the mechanical trauma caused by insertion and chronic presence of electrode structures, the entire electrode structure and associated wires must be as small as possible consistent with the required ability to conduct electrical energy, and must be of materials which will not react with the living body.
Implanted electrodes and conductors connected to them must be electrically insulated very effectively, because of the very small voltages and currents being utilized. The localized nature of the electrical potential gradient which must be detected by a microelectrode, and the fragility of neurons, dictate a microelectrode tip with small dimensions (typically less than 5.times.25 microns), which in turn produces a high impedance in the interface of metal to electrolyte. Since the probe as a whole must have a slender profile to minimize disruption of tissue, the requirement to minimize shunt losses along the insulated shank of the probe falls on a very thin dielectric coating which must be cleanly excluded from the tiny exposed tip or window. Insulating coatings on conductors must be free from pinholes and should be tightly adhered to the insulated wires and parts of electrodes. It is known that there are some biologically compatible dielectric materials which can be applied consistently and successfully as coatings of uniform thickness for such small structures as are found in microelectrodes to be used for neural prostheses. An insulating coating of Parylene-C.RTM., a polymerized diparachloroxylyene produced by the Union Carbide Corporation, is known to have the required biological compatibility and electrical insulation qualities and can be applied successfully to electrode surfaces, but the techniques previously available for removing portions of such a coating have not been entirely satisfactory.
At the same time, active contact sites of the electrodes must be clean and must typically present as low a resistance as is possible to electrical current at their surfaces. Although current is necessarily very small, because of the need to carry current pulses to very small regions, current density is significant in the small, exposed active contact site surface of an electrode, where it is exposed to the saline environment within a living body, and the electrode must be of a corrosion-resistant material to avoid electrochemical damage to the exposed surface of the electrode or the adjacent tissue through ion migration or other mechanisms.
Microelectrode tips require well defined active electrode sites for use as stimulation electrodes. A limiting factor in producing microelectrodes to be used in stimulating and receiving information from neurons is the ability to remove small areas of insulation cleanly and accurately, leaving clean electrode contact surfaces of limited size to be exposed to neurons. Various techniques for exposing portions of an electrode have been used in the past, but it is difficult to accurately reproduce the desired tip exposure using them. Such techniques have included AC electric corona arcing, direct heating, and plasma etching. These methods have not been completely satisfactory, either because they fail to leave a cleanly and accurately exposed electrode surface, or because the remaining adjacent insulating coating does not adhere satisfactorily and tightly to the microelectrode adjacent the exposed surfaces. Mechanical removal of an insulating coating has been very time-consuming and has a high probability of damaging the tip.
Multiple conductor microelectrodes have been produced using photolithographic integrated circuit production techniques, but these are not robust enough for some applications, and are very expensive to produce in small numbers. Since they are produced as small integrated circuits they lack conductors for connection to other electrical circuitry. It is difficult to attach conductors to such devices and then protect surfaces in the vicinity of such connections to prevent undesired electrical activity when implanted.
Use of lasers to pierce dielectric coatings in preparation of microelectrodes was described by M. J. Mela in 1965 in an article entitled "Microperforation with Laser Beam in the Preparation of Microelectrodes," published in IEEE Transactions on Biomedical Engineering, Vol. BME-13, No. 2, pp. 70-76. Mela disclosed use of a red light ruby laser, which does not satisfactorily clean insulating coatings from metal surfaces, as is needed for suitably low surface resistance. That is, before the present invention it has not been known how to remove biologically compatible dielectric materials cleanly from a metal surface using a laser to produce well-defined surface areas for contact in order to achieve a well-defined surface resistance.
What is still needed, then, is a microelectrode and a method for manufacturing such a microelectrode which is suitable for chronic biological implantation, which defines contact surfaces cleanly exposed, of an accurately predetermined and controlled size and location, and surrounded by effectively and securely attached dielectric material.